1. Field of the Invention
The present invention pertains to Laser Detection and Ranging (“LADAR”) systems, and, more particularly, to linear array LADAR systems.
2. Description of the Related Art
A need of great importance in military and some civilian operations is the ability to quickly detect and identify objects, frequently referred to as “targets,” in a “field of view.” A common problem in military operations, for example, is to detect and identify targets, such as tanks, vehicles, guns, and similar items, which have been camouflaged or which are operating at night or in foggy weather. It is important in many instances to be able to distinguish reliably between enemy and friendly forces. As the pace of battlefield operations increases, so does the need for quick and accurate identification of potential targets as friend or foe, and as a target or not.
Techniques for identifying targets have existed for many years. For instance, in World War II, the British developed and utilized radio detection and ranging (“RADAR”) systems for identifying the incoming planes of the German Luftwaffe. RADAR uses radio waves to locate objects at great distances even in bad weather or in total darkness. Sound navigation and ranging (“SONAR”) has found similar utility and application in environments where signals propagate through water, as opposed to the atmosphere. While RADAR and SONAR have proven quite effective in many areas, they are inherently limited by a number of factors. For instance, RADAR is limited because of its use of radio frequency signals and the size of the resultant antennas used to transmit and receive such signals. Sonar suffers similar types of limitations. Thus, alternative technologies have been developed and deployed.
One such alternative technology is laser detection and ranging (“LADAR”). Similar to RADAR systems, which transmit and receive radio waves to and reflected from objects, LADAR systems transmit laser beams and receive reflections from targets. Because of the short wavelengths associated with laser beam transmissions, LADAR data exhibits much greater resolution than RADAR data. Typically, a LADAR system creates a three-dimensional (“3-D”) image. The reflected LADAR signal provides an (x,y) coordinate for the point that reflected the signal. The time of flight of the LADAR signal (i.e., the time lapse between the transmission of the LADAR signal and the receipt of its reflection) is used to determine the range to the point of reflection, which provides a z-axis coordinate. Each datum comprising an (x,y) coordinate and associated range is referred to as a “pixel.”
The pixel resolution is a function of their spacing, and is fixed by the optical system and/or, for scanned systems, the scan rate. Many LADAR systems require a nominal pixel spacing to maintain automatic target recognition (“ATR”) performance. In fact, for greater ATR performance, it is desired that the pixel spacing remained fixed over the system's range capabilities. Thus, for a fixed pixel spacing, the LADAR system's ATR performance is a function of the LADAR system's range. For systems where the minimum and maximum ranges differ by one, two, or more orders of magnitude, this requires the sensor to optically change the field of view (“FOV”) to meet the challenge of maintaining a fixed pixel spacing. For instance,
To date, the problem has been solved in several ways, including:                (i) optical FOV changes;        (ii) interlacing returns to give two effective FOVs, as is disclosed in U.S. Pat. No. 5,898,483, entitled “Method for increasing LADAR resolution”, issued Apr. 27, 1999, to Lockheed Martin Corp. as assignee of the inventor E. Max Flowers, and commonly assigned herewith; and        (iii) a staggered linear array to give two FOVs, as is done in Polarimetric Imaging Laser Radar (“PILAR”) on various unmanned aerial vehicles (“UAVs”) deployed by the United States military services.Thus, multiple detector imaging sensors have fixed detectors. With fixed detectors, multiple FOV optics (or zoom lens) are required to maintain image resolution (spatial pixel spacing) at different ranges. LADAR sensors are no different. In fact, multiple beam/detector LADAR sensors are even more difficult to design with variable pixel spacing due to the transmitter receiver alignment for each required FOV.        
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.